'My experience is one controller will stop charging before tho other."
Same here. If it were me, I'd strip the controller box out of one suitcase and wire those panels' outputs directly to the inputs of the other suitcase's controller. That way you only have one controller for all your panels.
A better approach: sell that suitcase to somebody else and just get a couple of [URL="https://amzn.to/2C6D6A7"]100 W Renogy (note spelling)
panels [URL="https://amzn.to/2VBtCG4"] and hook 'em up. You'll get the same amount of power for half the cost. If you wanted to, you could alway use piano hinge to connect the two, and make your own solar suitcase. .;-)
The solar suitcases are intended for people who just want a plug-and-play solution, and have no intention of ever upgrading to more panels. From the sound of it, that isn't you.